1. Field of the Invention
The present invention relates in general to a structure and fabricating method for semiconductor memory. In particular, the present invention relates to a structure and fabricating method for split gate flash memory.
2. Description of the Related Art
A flash memory device is a non-volatile memory, derived from erasable programmable read-only memory (EPROM) and electrically-erasable programmable read-only memory (EEPROM). Flash memory is being increasingly used to store execution codes and data in portable electronic products, such as computer systems.
A typical flash memory comprises a memory array having a large number of memory cells arranged in blocks. Each of the memory cells is fabricated as a field-effect transistor having a control gate and a floating gate. The floating gate is capable of holding a charge, and is separated, by a layer of thin oxide, from source and drain regions contained in a substrate. Each of the memory cells can be electrically programmed (charged) by injecting electrons from the drain region through the oxide layer onto the floating gate. The charge can be removed from the floating gate by tunneling the electrons to the source through the oxide layer during an erase operation. Thus the data in a memory cell is determined by the presence or absence of a charge on the floating gate.
Stacked-gate and split-gate types are typically used in the memory cell structures of the flash memory.
Since the stacked-gate type memory cell structure is the same as the standard memory cell structure of the EPROM, it is advantageous for size reduction. However, there is a disadvantage in that the operational characteristics of the flash deteriorate during an erase operation due to overerase.
The split gate flash memory, in which the control gate includes a first portion overlaying a floating gate and a second portion directly overlaying the channel, is not susceptible to overerase problems. However, the size of the split gate flash memory cell is not small enough to achieve sufficiently reduced production costs.